Teeth that have lost functions due to caries or an accident or the like are restored by fixing a crowning restorative material made of a metal or ceramics, so-called inlay or crown, to the teeth, and an adhesive agent called a cement for dental use has been used in the fixation of the crowning restorative material to the teeth. Usually, upon adherence of the dentine and a crowning restorative material with a cement for dental use, a cement for dental use is applied in a slight excess amount to an inner wall side of the crowning restorative material to be pressed against the dentine. During this pressing procedure, a method including the steps of allowing an excess of a cement for dental use to be pushed out from a bonding part of the dentine and a crowning restorative material (hereinafter also referred to as a marginal part), and removing the pushed-out excess cement is employed. Therefore, the cement for dental use is provided as a paste-like composition having a high fluidity in a manner that the cement can be easily applied to the crowning restorative material and the excess cement is appropriately pushed out from the marginal part. In addition, unless the excess cement is completely removed, not only it is poor in esthetic appreciation but pushed-out and cured cement has a possibility of damaging the tissues in the oral cavity. Usually, this excess cement is removed by using a dental probe, or the like; however, it is difficult to remove the cement with the probe when the cement is in a state of high fluidity without being cured at all. Therefore, the removal of the excess cement is performed in a state in which the cement is in a completely cured state or in a state that the curing is progressed so that the fluidity is lost to a certain extent (semi-cured state).
Cements for dental use are classified into plural kinds depending upon the components and curing style, such as glass ionomer cement, resin-modified glass ionomer cement, and resin cement, each of which is actually used.
A glass ionomer cement is composed of a powder that elutes polyvalent metal ions and an aqueous solution of a polycarboxylic acid, and the cement is cured by chelate-crosslinking of the eluted polyvalent metal ions and the polycarboxylic acid upon mixing this powder and the aqueous solution. The cement takes a simple procedure because a pretreatment of the dentine is generally unnecessary, and has the feature that has excellent removability of an excess cement. However, the reason why the glass ionomer-cement has excellent removability of an excess cement is in that mechanical strength of the completely cured cement is low as compared to that of the resin cement, thereby making it possible to easily breakdown the cured-cement upon the removal of the cement using a probe. Therefore, while the glass ionomer cement has the advantages mentioned above, the cement has a disadvantage in the aspect of durability (reliability) of the cement itself. Further, the glass ionomer cement has a disadvantage that its physical properties such as mechanical strength lower upon contact with water such as saliva during curing are lowered.
In order to overcome the disadvantages owned by the glass ionomer cement, in the recent years, a cement for dental use called a resin-modified glass ionomer cement in which a radical-polymerizable monomer and a chemical polymerization initiator are blended, in addition to a polycarboxylic acid, is developed and made available to the market. The cement has improved mechanical strength, which has been a disadvantage in the glass ionomer cement, by curing according to radical-polymerization in addition to curing according to chelate-crosslinking, thereby allowing the polymer of the radical-polymerizable monomers to be present in the resulting cured product. However, the resin-modified glass ionomer cement as described above is no different in the aspect that the cured product is mainly made of a polycarboxylic acid and a chelating compound of a polyvalent metal ion (ionomer), so that the cement has a low mechanical strength and insufficient reliability, as compared to the resin cement mainly made of the polymer of the radical-polymerizable monomers. In addition, recently, a resin-modified glass ionomer cement, which is mainly made of a polymer of radical-polymerizable monomers, having a mechanical strength even more closer to that of a resin cement is used for practical purposes; however, the cement for dental use as described above has some disadvantages that a time period in a semi-cured state is so short that timing for removing the cement is made difficult, and that the mechanical strength becomes too high if completely cured, so that it is very difficult to remove the cement, as in the same manner as the resin cement described later.
Among the cements for dental use, a resin cement is composed of a composition containing a radical-polymerizable monomer, an inorganic or organic filler, and a chemical polymerization initiator, in which curing is carried out by radical polymerization. In addition, since a radical-polymerizable monomer containing an acidic group is blended as a part of the radical-polymerizable monomers, the resin cement shows a firm adhesion to the dentine and various metals.
Also, a resin cement in which an inorganic filler is mainly blended as a filler has even more excellent mechanical strength and durability. In the cement having a high mechanical strength as described above, since it is difficult to remove an excess cement after the cement is completely cured, the removal of an excess cement is carried out in a semi-cured state. However, a chemical polymerization initiator is blended in the cement in an amount that enables complete curing of the cement, so that there is a disadvantage that a time period for which the cement is in a semi-cured state is short, thereby making the timing for removing the cement difficult.
On the other hand, in a resin cement blended mainly with an organic filler as a filler, a cement cured product becomes undesirably elastic, so that it is again difficult to remove the cement with a probe after being completely cured. Therefore, the removal of an excess cement is carried out in a semi-cured state; however, there is a disadvantage that a time period for which the cement is in a semi-cured state is short, thereby making the timing of removing an excess cement difficult, as in the same manner as that blended with an inorganic filler.
Furthermore, in a resin cement being valued of its high bond strength against the dentine or a crowning restorative material, regardless of using an inorganic filler or an organic filler, if an excess cement that is deposited to sites other than a desired site is undesirably completely cured and firmly adhered, it is very difficult to remove the cured product.
On the other hand, in a cement for dental use in which radical-polymerizable monomers for a resin cement or the like are main curing components, techniques of adjusting a chemical curing time, thereby improving removability of an excess cement in chemical curing have been disclosed.
For example, Patent Publication 1 discloses a technique of delaying a curing time by adding a polymerization inhibitor, and Patent Publication 2 discloses a technique of improving removability of an excess cement in chemical curing by blending a styrenic derivative having a specified structure, thereby extending an operating time from the beginning of curing to the termination of curing.
On the other hand, in the recent years, in a cement for dental use in which radical-polymerizable monomers for a resin cement or the like are main curing components, a dual curing material having both photocuring and chemical curing properties, the dual curing material containing a photopolymerization initiator in addition to a chemical polymerization initiator, is widely used (Patent Publication 3).    Patent Publication 1: Japanese Patent Laid-Open No. Hei 9-67222    Patent Publication 2: WO 2003/057180    Patent Publication 3: Japanese Unexamined Patent Publication No. 2004-529946